Human sex ratio

In anthropology and demography, the human sex ratio is the ratio of males to females in a population. More data are available for humans than for any other species, and the human sex ratio is more studied than that of any other species, but interpreting these statistics can be difficult.

Like most sexual species, the sex ratio in humans is approximately 1:1. The sex ratio at birth worldwide is commonly thought to be 107 boys to 100 girls, although this value is subject to debate in the scientific community. The sex ratio for the entire world population is 101 males to 100 females. Depending upon which definition is used, between 0.1% and 1.7% of live births are intersex.

Gender imbalance may arise as a consequence of various factors including natural factors, exposure to pesticides and environmental contaminants, war casualties, gender-selective abortions and infanticides, aging, and deliberate gendercide.

Human sex ratios, either at birth or in the population as a whole, are reported in any of four ways: the ratio of males to females, the ratio of females to males, the proportion of males, or the proportion of females. If there are 108,000 males and 100,000 females the ratio of males to females is 1.080 and the proportion of males is 51.9%. Scientific literature often uses the proportion of males. This article uses the ratio of males to females, unless specified otherwise.

In a study around 2002, the natural sex ratio at birth was estimated to be close to 1.06 males/female. In most populations, adult males tend to have higher death rates than adult females of the same age (even after allowing for causes specific to females such as death in childbirth), both due to natural causes such as heart attacks and strokes, which account for by far the majority of deaths and also to violent causes, such as homicide and warfare (for example, in the USA as of 2006, an adult non-elderly male is 3 to 6 times more likely to become a victim of a homicide and 2.5 to 3.5 times more likely to die in an accident than a female of the same age), resulting in higher life expectancy of females. Consequently, the sex ratio tends to reduce as age increases, and among the elderly there is usually an excess of females. For example, the male to female ratio falls from 1.05 for the group aged 15 to 65 to 0.70 for the group over 65 in Germany, from 1.00 to 0.72 in the USA, from 1.06 to 0.91 in mainland China and from 1.07 to 1.02 in India.

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